In certain known types of electronic systems, particularly those designed for electronic article surveillance, it is known to provide a composite antenna comprising two or more antennas coupled to each other in one way or another, and to which signals from a transmitter are supplied so as to produce an induction field adjacent the composite antenna which is sufficiently strong to detect the presence near the antenna of predetermined types of objects; in order to avoid the production of relatively strong far fields which might interfere with the operation of other electronic apparatus, it is known to design such composite antennas so that their net effect at positions remote from the antennas is substantially zero, or at least insufficient to cause any serious problem.
A particular type of system with respect to which the present invention will be described in detail is an electronic article surveillance system of the type in which a tag or other electronically detectable marker is secured to articles to be protected against unauthorized removal from protected premises, and in which the exits from the premises through which the goods would normally be removed are irradiated by a transmitted field from an antenna system; the response of the marker to such transmitted fields is then detected by an appropriate nearby receiver. In one wellknown form of such system, the marker is a tag circuit on a small tag secured to the article to be protected, which circuit resonates in response to the signals transmitted by the antenna, thereby producing return signals at the receiver which indicate the presence of the tag and the article to which it is attached.
In order to provide the desired far-field cancellation, it is known to constitute the antenna of a plurality of loop antennas the planes of which are substantially parallel and adjacent but displaced from each other, and in which the direction of transmitter current flow with respect to the environment is opposite in different loops, so that the remote fields produced at any remote point by the loops are opposite in phase with respect to the environment. Using such a composite antenna, it has been found possible to cancel the far field substantially completely by suitable choice of the cross-sectional areas and numbers of turns in the several loop antennas.
In one simple form, for example, such a composite antenna may comprise two loop antennas formed from the same continuous wire by, in effect, twisting the two halves of the antenna by 180.degree. to produce a configuration analogous to a FIG. 8; in such an antenna, the directions of flow of the currents at any instant are opposite with respect to the environment, and if the two loops have the same number of turns and the same area, substantially complete cancellation of far fields will be effected. More than two such loops may be employed in accordance with the prior art, with the same intensity of current and the same number of wires in each loop, and with the total area of the loops operating in a given phase equalling the total area of the loops operating in the opposite phase.
Although the far-field effects of the composite antenna are then substantially cancelled, the magnetic "near-fields" due to the respective loop antennas may differ substantially from each other, depending upon exactly where the article to be detected is located. For example, if the article is located nearly in alignment with the center of one of the loops and near it, it will be affected primarily by the transmitter signal radiated by that loop and if it is aligned with, and near, the center of another of the loops, it will be affected primarily by the transmitter signal in that loop. Thus, cancellation of the near field will not occur in either of the latter specified circumstances , and in fact near-field cancellation normally occurs only in a relatively small region. It is the non-cancellation of the near field in most of the region near the transmitter antenna which permits detection of the protected object, as is desired.
However, as noted above, in general there will be some limited regions in the RF induction near-field adjacent the antenna in which the transmitted signal components from the various loops of the composite antenna do substantially cancel each other; for example, in the case of two loops of equal area and equal but opposite current intensity, each using the same number of wires in its loop, a substantial null in the near field will exist in and near a plane at right angles to the plane of the loops and passing through a mid-point between them.
While such near-field nulls cannot be completely eliminated, it has been possible to control to some extent their locations. The positions at which such null regions can best be tolerated depends on the particular application of the system, and it is generally desirable to be able to design the antenna system to avoid such nulls at certain positions where article-detection is important.
For example, in the case of vertically disposed antenna loops positioned one above the other adjacent the path along which customers leave protected store premises, it is possible to utilize one loop antenna operating in a particular phase and of large cross-sectional area extending, for example, from two to five feet above the floor, so that articles removed past the antenna in most of this height range will be readily detected, and to utilize an oppositely-phased loop above and an oppositely phased loop below the principal central antenna to provide the desired far-field cancellation as well as additional detection at very low and very high levels. In such case, for example, the near-field null regions will be limited to positions near the two foot and five foot levels, so that an article hidden on the person or carried in a bag above the knees and below the shoulders, or in a very high or very low position, is likely to be detected. However, this may not be the optimum position for the near-field nulls in all cases, and the length of wire used in the antenna also may not be optimum; it should be recognized that in the type of systems specifically described hereinafter, the more wire length utilized in the antenna, the more undesired resonant frequencies arise in the antenna system, and if too much wire is employed such resonances may, in fact, lie within the operating bandwidth of the wide-bandwith RF EAS system and interfere with its operation. Accordingly, it is also generally desirable to minimize the number of loops and the number of turns per loop in the antenna system.
Aside from the problem of the location of the null regions, there is the problem of controlling the configuration of the net near-field strength adjacent the antennas so that the higher field strengths occur in the region where they are most helpful. It will be understood that tag circuits in some locations and orientations near the antennas respond less strongly to the radiated near field than do tag circuits in other location and/or orientations, and therefore require higher near-field strengths to assure their detection. Increasing the radiated power proportionally in all directions so as to assure detection of such hard-to-detect tags would be wasteful of power, and likely to result in unacceptably high remanent far-field strengths, even though they may be minimized by the cancellation technique described above. What is desirable is to enhance selectively the field strengths in the regions where tag detection is expected to be difficult.
Unfortunately, as pointed out above, one is constrained, in varying the loop areas and the number of turns on the various loops, by the need to maintain adequate far-field cancellation and the desirability of using only integral numbers of turns in the loops and as little antenna conductor length as possible.
It will therefore be appreciated that there are a variety of considerations involved in selecting the optimum antenna system for any particular application, not all of which can readily be met by mere selection of the areas of the loops, the number of loops and the number of turns in each loop, nor even by selection of the geometric shape and positioning of the loops.
Accordingly, it is an object of the present invention to provide a new and useful composite antenna system of the type utilizing a plurality of antennas to produce a substantial net near field adjacent the antennas, but very low or near-zero net far-field strengths at positions remote from the antenna.
Another object is to provide such a composite antenna which provides a greater choice of design parameters than do previously-known composite antennas.
A still further object is to provide such a composite antenna which enables concentration of the field intensity in regions where they are most needed to detect hard-to-detect tags, and which also enables control of the location of the near-field null regions, without requiring an excessive number of antenna loops or number of turns in each loop and without producing excessive net far-field strengths.